1. Field of the Invention
The present invention relates to an optical scanning device and an image forming apparatus.
2. Description of the Related Art
Image formation by optical scanning is widely implemented in various image forming apparatuses such as a digital copier and a laser printer. As an optical scanning method for enabling a high-speed image formation by optical scanning, a multi-beam scanning method is known, and as a laser light source appropriate for this scanning method, recently, a vertical-cavity surface-emitting laser (VCSEL) has been attempted for use.
In an edge emitting semiconductor laser array that has been conventionally known as a laser light source for the multi-beam scanning method and a beam combination by using a plurality of edge emitting semiconductor lasers and a beam combining prism, the number of laser emission sources that can be simultaneously used is limited to a small number.
On the other hand, the VCSEL can array tens to hundreds laser emission sources within the same surface from which a plurality of laser beams is emitted, thereby enabling to perform the optical scanning on tens to hundreds scanning lines at the same time, enabling to fully utilize an advantage of multi-beam scanning, i.e., high-speed image formation.
However, when the VCSEL is used as a laser light source, the following problems occur.
That is, although the laser beam emitted from the VCSEL is linearly polarized light beam, when a plurality of laser emission sources is allocated in the same VCSEL, it is difficult to align the polarization direction of the laser beam emitted from each laser emission source. Therefore, it is difficult to obtain a plurality of laser beams with the same polarization direction.
In addition, it is known that the polarization direction of the laser beam emitted from the VCSEL changes with the passage of time; therefore, even if a plurality of laser beams with the same polarization direction can be realized, the polarization direction of each laser beam may vary with the passage of time.
As is commonly known, reflectance of light in a linearly polarized state greatly changes depending on a reflection angle. For this reason, with the change of incident angle onto the deflection reflection surface of a polygon mirror that deflects the laser beam, an incident angle onto the lens face of an fθ lens, and other factors, the amount of the laser beam that reaches a scanning surface changes. Correction of such change of the amount of laser beam that reaches the scanning surface has been conventionally known as shading correction.
Because the change of the reflectance of light in a linearly polarized state by reflection angle differs between P polarization component and S polarization component, as a conventionally known shading correction, it is known that the laser beam emitted from the laser source is changed to circularly polarized light or elliptically polarized light before the laser beam enters the deflection reflection surface.
In addition, unlike the edge emitting semiconductor laser, in the VCSEL, the laser beam is emitted only from the light emitting surface thereof. For this reason, in order to control the light intensity, it is necessary to separate a part of emitted laser beam, guides it in a sensor for controlling the light intensity, and control the light intensity in accordance with the output of the sensor.
Because a semitransparent mirror is generally used for separating the laser beam, if the reflectance in the semitransparent mirror changes due to the change of the polarization direction, there is a possibility that control of the light intensity may be performed in accordance with the change of the reflectance thereof, thereby causing also an error in the shading correction.
An optical scanning device that is studied in view of such points and an apparatus described in Japanese Patent Application Laid-open No. 2005-156933 are known.